CRAYFISH BURROWS FROM THE LATEST CRETACEOUS LOWER CANTWELL FORMATION (DENALI NATIONAL PARK, ALASKA): THEIR MORPHOLOGY AND PALEOCLIMATIC SIGNIFICANCE
The Cantwell Formation fills the Cantwell Basin, a 135 km-long and up to 35 km-wide, east–west trending basin, bracketed by the Hines Creek Fault to the north and the McKinley Fault to the south. Basin fill comprises up to 4000 m of continental deposits, predominantly including braided rivers, alluvial fans, floodplains, and ponds.
Crayfish burrows from the Lower Cantwell Formation, a Cretaceous high-latitude paleoecosystem, provide evidence of moisture fluctuations, as well as insight into the mean annual temperature experienced at the time. The Cretaceous mean annual temperature was more like that of southern-most Ontario, Canada, where the northern-most burrowing crayfish are found today. The burrow depth suggests (1) no permafrost was present and (2) the phreatic zone was ~30–50 cm below the paleo-ground surface. The paleoclimate is interpreted as humid continental (Köppen scheme) with average summer high temperatures between 25oC to 28oC and average low temperatures between -6oC to 0oC, estimates that compare somewhat favorably with previous CLAMP estimates of a warm monthly mean of 17.08 +/- 1.6 oC and a cold monthly mean of -2.31 +/- 1.9 oC.